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Glucocorticoids

Effects on Gene Transcription

Department of Thoracic Medicine, National Heart and Lung Institute, Imperial College, London, United Kingdom

Correspondence and requests for reprints should be addressed to Ian M. Adcock, Ph.D., Department of Thoracic Medicine, National Heart and Lung Institute, Dovehouse St, London SW3 6LY, UK. E-mail: ian.adcock{at}ic.ac.uk

	ABSTRACT

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

 
The major antiinflammatory effects of glucocorticoids appear to be due largely to interaction between the activated glucocorticoid receptor and transcription factors, notably nuclear factor-B (NF-B) and activator protein-1, that mediate the expression of inflammatory genes. NF-B switches on inflammatory genes via a process involving recruitment of transcriptional coactivator proteins and changes in chromatin modifications such as histone acetylation. This process must occur in the correct temporal manner to allow for effective inflammatory gene expression to occur. Glucocorticoids, using a similar mechanism, are also able to switch on a number of antiinflammatory genes. An important question is why glucocorticoids switch off only inflammatory genes, as they clearly do not suppress all activated genes and are well tolerated as long-term treatments. The interactions between NF-B and the glucocorticoid receptor result in differing effects on histone acetylation and deacetylation. Oxidative stress due to cigarette smoke may be an important factor in inducing glucocorticoid resistance in chronic obstructive pulmonary disease and may involve changes in histone acetylation/deacetylation balance.

Key Words: asthma • chronic obstructive pulmonary disease • histone acetylation/deacetylation • glucocorticoid resistance • nuclear factor-B

	THE MOLECULAR BASIS OF INFLAMMATION IN BRONCHIAL ASTHMA

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

 
Inflammation is a central feature of many chronic lung diseases, including bronchial asthma and emphysema. The specific characteristics of the inflammatory response and the site of inflammation differ between these chronic lung diseases, but all involve the recruitment and activation of inflammatory cells and changes in the structural cells of the lung. These structural changes include basement membrane thickening, epithelial cell loss, airway smooth muscle hypertrophy, hyperplasia, and migration in asthma, and matrix destruction in emphysema. Asthma and chronic obstructive pulmonary disease (COPD), for example, are characterized by increased expression of many proteins involved in the complex inflammatory cascade, including cytokines, chemokines, receptors, and adhesion molecules (reviewed in Ref. 1).

The increased expression of most of these proteins is the result of enhanced gene transcription, because many of the genes are not expressed in normal cells under resting conditions but are induced in a cell-specific manner. Changes in gene transcription are regulated by proinflammatory transcription factors, such as nuclear factor-B (NF-B) and activator protein-1 (AP-1) (2). For example, NF-B is markedly activated in epithelial cells of patients with asthma (3), and it regulates many of the inflammatory genes that are abnormally expressed in asthma (4). NF-B may be activated by rhinovirus infection and allergen exposure, both of which exacerbate asthmatic inflammation (5, 6). AP-1 expression is also enhanced in asthmatic airways (7), and it is reduced after glucocorticoid therapy (9). This enhanced AP-1 expression is further elevated in severe steroid-insensitive asthma (8) resulting from increased expression or an insensitivity to downregulation by glucocorticoids. In addition, its expression and activity is enhanced by factors associated with asthma and airway hyperresponsiveness and remodeling (10, 11).

	CHROMATIN REMODELING

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

 
DNA is tightly compacted around a protein core. This chromatin structure is composed of nucleosomes, which are particles consisting of approximately 146 bp DNA associated with an octamer of 2 molecules each of core histone proteins (H2A, H2B, H3, and H4). Expression and repression of genes is associated with alterations in chromatin structure by enzymatic modification of core histones (12). In the resting cell, DNA is tightly compacted around these basic core histones, excluding the binding of the enzyme RNA polymerase II, which activates the formation of messenger RNA. This conformation of the chromatin structure is described as closed and is associated with suppression of gene expression (12) (Figure 1).

View larger version (36K): [in this window] [in a new window]   Figure 1. Gene repression and activation are regulated by acetylation of core histones. In the resting state, DNA is tightly coiled around histones, forming a dense nucleosomal structure due to electrostatic attraction between negatively charged DNA and positively charged lysine residues. Acetylation of histones removes this charge, allowing loosening of the nucleosomal structure. Histone acetylation is mediated by transcriptional coactivators, which have intrinsic histone acetyltransferase (HAT) activity, whereas repression is induced by histone deacetylases (HDACs), which reverse this acetylation, allowing repackaging of the nucleosomes. Adapted from Urnov and Wolffe (12).

	HISTONE ACETYLATION AND GENE TRANSCRIPTION

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

 
Specific residues (lysines, arginines, and serines) within the N-ter-minal tails of core histones are capable of being post-translationally modified by acetylation, methylation, ubiquitination, or phosphorylation, all of which have been implicated in the regulation of gene expression (12). Acetylation of the -group on lysines reduces the charge of the histone residue and subsequently releases the tightly wound DNA, allowing the recruitment of further large protein complexes (12). A breakthrough in the discovery of the role of histone acetylation was the demonstration that transcriptional coactivators such as CREB-binding protein (CBP) and p300/CBP-associated factor have intrinsic histone acetyltransferase (HAT) activity. This activity is therefore recruited to the site of active gene transcription by the binding of transcription factors to DNA. This association between coactivator and transcription factor may, in addition, further enhance coactivator HAT activity (12, 14).

Increased gene transcription is therefore associated with an increase in histone acetylation, whereas hypoacetylation is correlated with reduced transcription or gene silencing (12). Histone acetylation is an active process whereby small changes in the activity of HATs or histone deacetylases (HDACs) can markedly affect the overall histone acetylase activity associated with inflammatory genes (12). Importantly, these changes in histone acetylation appear to be targeted toward regions of DNA associated with specific activator sites within the regulatory regions of induced inflammatory genes (12), although a global loosening of histone structure has also been proposed (15).

Histone Deacetylation
Repression of genes is associated with the reversal of histone acetylation, or histone deacetylation, a process controlled by HDACs (16). The number of known HDACs is growing; so far, at least 11 mammalian forms have been identified along with at least 6 sirtuins (16). These HDACs are categorized into two classes according to homology with yeast HDACs. Class 1 HDACs (HDAC1, 2, 3 and 8) are most closely related to the yeast (Saccharomyces cerevisiae) transcriptional regulator RPD3. Class II HDACs (HDAC4, 5, 6, 7, 9, and 10) share domains with similarity to HDA1, another deacetylase found in yeast (16). Currently it is thought that HDACs of class I are expressed in most cell types, whereas the expression pattern of class II HDACs is more restricted, suggesting that they might be involved in cellular differentiation and developmental processes (16). Class II HDACs often shuttle between the nucleus and the cytoplasm and in the case of HDAC4 and 5 are important in cardiac myocyte development (16). The regulation of this shuttling process is linked to cellular kinase signaling networks changing HDAC phosphorylation and association with 14-3-3 docking proteins (16, 17). Deacetylation of histones increases the winding of DNA around histone residues, resulting in a dense chromatin structure and reduced access of transcription factors to their binding sites, thereby leading to repressed transcription of inflammatory genes (12).

However, the simple model described above does not tell the full story. Under resting conditions, less than half of the potential lysine residues available for acetylation are in fact acetylated, and these residues have a rapid turnover (15). This situation suggests that even small changes above or below the resting level are enough to lead to an activated chromatin state. Furthermore, this model predicts that changes in the "histone code" (18) must be translated into downstream events extremely rapidly (15). The "histone code" refers to the diverse range of histone tail post-translational modifications such as acetylation, methylation, phosphorylation, and ubiquitination, which are set and maintained by histone-modifying enzymes and contribute to coactivator recruitment and subsequent increases in transcription.

	NF-B

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

 
Although numerous different pathways are activated during the inflammatory response, NF-B is thought to be of paramount importance in asthmatic inflammation because it is activated by all the stimuli considered important in the inflammatory response to allergen exposure (19). In addition, it is a major target for glucocorticoids (4). NF-B is ubiquitously expressed within cells, and it not only controls induction of inflammatory genes in its own right but also enhances the activity of other cell- and signal-specific transcription factors (4, 19, 20).

NF-B is activated by numerous extracellular stimuli, including cytokines such as tumor necrosis factor (TNF)- and interleukin (IL)-1ß, viruses, and immune challenges (19). Activation of cell-surface receptors leads to phosphorylation of receptor-associated kinases (4), which in turn phosphorylate the inhibitors of NF-B kinase (IKKs). Phosphorylation of IKKs results in phosphorylation of the NF-B cytoplasmic inhibitor (IB), so that IB is targeted for proteosomal degradation. This degradation precipitates the release of NF-B from its inactive state, enabling nuclear translocation and binding to specific DNA response elements within the regulatory regions of responsive genes (4).

NF-B is predominantly composed of the p50/p65 heterodimer (19). Subtle changes in p65 phosphorylation are also influential; for example, inactive p65 is nonphosphorylated and is associated predominantly with HDAC1, whereas p65 is phosphorylated after IKK-2 stimulation and is able to bind to coactivator molecules such as p300/CBP (21).

NF-B Induces Histone Acetylation
Cytokines such as TNF- and IL-1ß, acting via NF-B, can induce histone acetylation in both a time- and concentration-dependent manner (22). This NF-B–induced acetylation occurs preferentially on histone H4, rather than histones H2A, H2B, or H3, and it is directed primarily toward lysine residues 8 and 12 at NF-B–responsive regulatory elements (22). It is clear that changes in H3 phosphorylation and acetylation also occur after NF-B activation, although to a lesser extent than acetylation on H4 in airway cells. It is not known whether modifications on H2A and H2B occur after NF-B association/activation. The "histone code" would suggest that even small changes in histone tail modifications could have marked structural changes and allow recruitment of distinct coactivator complexes. Upon DNA binding, NF-B recruits a large coactivator complex that contains the HAT proteins CBP and p300/CBP-associated factor. In overexpression experiments CBP and PCAF are involved in p65-associated increases in HAT activity, but it is not clear whether this is the case in "normal" cells (22). Immunoprecipitated p65 associated in-gel HAT assays shows that a specific, approximately 65 kD, HAT is activated after IL-1ß stimulation (K.I., unpublished observations). IL-1ß can also activate other pathways, distinct from NF-B, that can impinge on NF-B activation (23). These additional pathways, such as protein kinase C and nonreceptor tyrosine kinases, may enhance NF-B activity, either by phosphorylating p65 and thereby enhancing cofactor recruitment (21) or by phosphorylating NF-B–associated cofactors (23). Several other HATs have been reported to be associated with NF-B, including transcriptional intermediary factor-2 (TIF-2) (24), also known as glucocorticoid receptor interacting protein-1 (GRIP)-1; p300 (25); and members of the p160 family and steroid receptor coactivator-1 (SRC-1) (26).

In many studies phosphorylation of Serine 10 in histone H3 is associated with gene induction (12). IKK-1 is generally thought to be predominantly involved in innate immune responses rather than cytokine-induced NF-B function. However, two recent papers suggest that IKK-1 can modulate NF-B–dependent gene expression in response to TNF- treatment (27, 28). Based on chromatin immunoprecipitation assays, it was reported that IKK-1 is recruited to the promoters of NF-B–regulated genes in association with CBP and p65 after stimulation with TNF-. This resulted in gene-specific phosphorylation of serine 10 on histone H3 and subsequent increased gene expression.

	TEMPORAL ASSOCIATION OF NF-B WITH DNA, COFACTORS, AND GENE INDUCTION

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

 
In a series of studies using chromatin immunoprecipitation assays, Saccani and colleagues have shown that the model described here previously needs modification (29, 30). This simple model predicts that activation of NF-B results in NF-B binding to B sites in the regulatory regions of inflammatory genes, subsequent recruitment of HATS, and chromatin modification leading to chromatin remodeling and induction of gene transcription. Immediate early genes such as IB do indeed bind NF-B to their promoters rapidly after lipopolysaccharide (LPS) stimulation, but within 10 minutes NF-B dissociates from the IB promoter site and never reassociates. In contrast, NF-B binds to its promoter sites in DNA for up to 2 hours before dissociation in distinct sets of genes (manganese superoxide dismutase and macrophage inflammatory protein-2), in spite of stimulation by LPS at the same time. However, other NF-B–regulated genes, such as the chemokine regulated on activation, normal T cell expressed and secreted (RANTES), monocyte chemoattractant protein-1, and IL-6, do not show NF-B binding to their promoters until 2 hours after activation. NF-B sites in the promoter regions of these genes are originally in a repressed chromatin environment that prevents NF-B DNA binding and subsequent gene expression. These become accessible only after AP-1–mediated histone acetylation and subsequent alteration in the local nucleosomal structure (29, 30) (Figure 2). Thus, there are subtle changes in NF-B DNA binding that are promoter context–dependent, precede coactivator recruitment, and are not detectable using conventional band shift and reporter gene assays.

View larger version (17K): [in this window] [in a new window]   Figure 2. Nuclear factor-B (NF-B) activation of inflammatory genes. (A) Simple model of NF-B activation of IB gene expression. Activation of NF-B by exogenous stimuli results in NF-B binding to B sites in the regulatory regions of the IB promoter, subsequent recruitment of HATs, and chromatin modification, leading to chromatin remodeling and induction of gene transcription (txn). In the case of IB, NF-B is rapidly ejected from DNA within 10 minutes. (B) A more complex model accounts for the fact that many genes, such as monocyte chemoattractant protein-1 (MCP-1), do not show NF-B promoter binding for up to 2 hours after lipopolysaccharide (LPS) stimulation of NF-B nuclear translocation, and no transcription of these genes occurs during this time. MCP-1 transcription requires AP-1 binding to its consensus site within the MCP-1 regulatory domain and subsequent ATP-dependent chromatin remodeling using the SWI/SNF complex to reveal the NF-B DNA binding site. Once the regulatory regions are remodeled, NF-B is able to bind to DNA, recruit HATs, and drive MCP-1 gene expression.

	GLUCOCORTICOID-INDUCED GENE TRANSCRIPTION

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

Glucocorticoids produce their effect on responsive cells by activating the GR to directly or indirectly regulate the transcription of target genes (31). The number of genes per cell directly regulated by glucocorticoids is estimated to be between 10 and 100, but many genes are indirectly regulated through an interaction with other transcription factors and coactivators. It seems highly unlikely that the widespread antiinflammatory actions of glucocorticoids could be explained by increased transcription of small numbers of antiinflammatory genes, such as annexin-1, IL-10, and the inhibitor of NF-B, IB (Table 1). However, therapeutic doses of inhaled glucocorticoids have not been shown to increase annexin-1 concentrations in bronchoalveolar lavage fluid (32), and an increase in IB has not been shown in all cell types (33).

View larger version (60K): [in this window] [in a new window]   Figure 3. How glucocorticoids switch on antiinflammatory gene expression. Glucocorticoids bind to cytoplasmic GRs that translocate to the nucleus, where they bind to GREs in the promoter region of glucocorticoid-sensitive genes. This leads to recruitment and activation of transcriptional coactivator molecules such as CBP, SRC-1, or other cofactors that have intrinsic HAT activity. This, in turn, results in acetylation of specific lysine residues on core histone proteins. Chromatin modification leads to local unwinding of the DNA structure, allowing recruitment of large protein complexes, including RNA polymerase II (RNA pol II). It is unclear whether chromatin remodeling by SWI/SNF is essential for this process. The process results in activation of genes en-coding antiinflammatory proteins, such as secretory leukoprotease inhibitor (SLPI), ß2-adrenergic receptors, and CD163. As an added complication, it is now clear that DNA binding by the GR is transient, lasting only seconds, whereas the subsequent changes are prolonged, suggesting epigenetic memory of the GR–GRE binding.

The question arises: how can the GR, or any other transcription factor, interact with its recognition site when DNA is compacted? The GR may bind to a GRE within the linker DNA between nucleosomes or, alternatively, when the GRE is wound around histones, as long as the core residues are facing outward (37). Binding to the GRE may then modify the local chromatin structure, altering GR access. High-resolution mapping of GR interactions with the MMTV-long terminal repeat (MMTV-LTR) in Xenopus oocytes suggests that the GR not only reorganizes the chromatin immediately surrounding its binding site but also can have effects elsewhere, thereby enforcing a particular translational frame on the chromatin template and modifying the effects of other DNA-binding proteins (38).

One other important question that needs to be addressed is whether there is a specific order of recruitment of distinct factors to the activated GR complex to gene transcription. Recent work examining androgen and thyroid hormone receptor gene activation shows that nuclear hormone receptors do not in themselves recruit all the cofactors required at the target promoters. Steroid receptor coactivators, recruited by receptors, can in turn recruit other coactivators, such as p300/CBP, and p300/CBP can subsequently recruit SWI/SNF (a large multi-subunit protein complex) and mediator complexes. SWI/SNF enables chromatin remodeling to occur in an adenosine triphosphate (ATP)-dependent manner, but histone acetylation by p300/CBP facilitates the recruitment of SWI/SNF and mediator complexes. Thus, cofactor–cofactor interactions are essential for effective gene expression. The interactions do not have to occur sequentially, but histone acetylation can enhance the recruitment of large multiprotein complexes in a coordinated manner (39).

However, again the story must be more complex. The intriguing data of Hager and colleagues provide clear evidence in vitro that the GR has a "hit-and-run" mechanism of action rather than a stable association with GRE (40). This group used fluorescence recovery after photobleaching and fluorescence loss in photobleaching to examine green fluorescent protein/GR association with a multimer of 200 copies of stably integrated MMTV-LTR. The results showed that the GR resided on DNA for less than 10 seconds before being ejected and replaced by another GR. This ejection may allow binding of additional regulatory factors that enhance gene transcription, such as HAT-containing complexes, and may also play a role in feedback regulation. Interestingly, in the absence of ATP and chromatin remodeling factors, the GR stably interacts with the MMTV-LTR (41).

	SWITCHING OFF INFLAMMATORY GENES

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

 
Cross-Talk between the GR and Other Transcription Factors
In spite of the ability of glucocorticoids to induce gene transcription, the major antiinflammatory effects of glucocorticoids are through repression of inflammatory and immune genes. The inhibitory effect of glucocorticoids appears to be due largely to interaction between the activated GR and the transcription factors, such as NF-B and AP-1, that mediate the expression of these inflammatory genes (31).

An important question is why glucocorticoids switch off only inflammatory genes, as they clearly do not suppress all activated genes and are well tolerated as long-term treatments. It is possible that the GR, acting as a monomer, binds only to specific coactivator complexes that are activated by proinflammatory transcription factors, such as NF-B and AP-1, although we do not understand how this specific recognition occurs. It is possible that the required residency time of GR on GRE may be a factor in distinguishing transactivation from transrepression. In this model low concentrations of glucocorticoids leads to fewer activated GRs having enough residency time on DNA to recruit coactivator complexes and so transcription does not occur. In contrast, this requirement for residency does not affect the association between p65 and GR, which can therefore occur at low concentrations.

The interaction between proinflammatory transcription factors and the GR may result in differing effects on histone acetylation/deacetylation, through one of several mechanisms that are probably not exclusive. The repressive action of glucocorticoids may be due to competition-activated GR binding to one of several transcription corepressor molecules, such as nuclear receptor interacting protein-1 and nuclear receptor corepressor-1, which associate with proteins that have differing histone deacetylase activity (34). In addition, IL-1ß and TNF- can cause histone acetylation on lysines 8 and 12 of histone H4, and low concentrations of dexamethasone (> 10^–9 M) can repress this IL-1ß-stimulated histone acetylation. This occurs by direct inhibition of CBP-associated HAT activity and by active recruitment of HDAC proteins (22) (Figure 4). In addition, high concentrations of glucocorticoids can induce HDAC expression in a time-dependent manner (22). Overall, this process results in the deacetylation of histones and repression of inflammatory genes (22).

View larger version (69K): [in this window] [in a new window]   Figure 4. How glucocorticoids switch off inflammatory gene expression. Many inflammatory genes are activated by stimuli, such as interleukin (IL)-1ß or tumor necrosis factor (TNF)-, and activate NF-B, which translocates to the nucleus. NF-B binds to specific B recognition sites in the promoter regions of responsive genes and subsequently recruits transcriptional coactivators, such as CBP or p300/CBP-associated factor, that have intrinsic HAT activity. This results in acetylation of lysines in core histones, leading to recruitment of large protein complexes, including RNA polymerase II (RNA pol II), and in turn leading to increased transcription of inflammatory genes (txn). Glucocorticoid receptors translocate to the nucleus after activation by corticosteroids and act as monomers, reducing histone acetylation. The activated GR monomer interacts with and inhibits the HAT activity of coactivator complexes. In addition, the GR is able to recruit HDAC to the NF-B complex, leading to suppression of inflammatory genes (no txn). Furthermore, the GR may also be able to reduce phosphorylation of serine 2 residues within the C-terminal repeat region of RNA polymerase II, reducing its capacity to cause mRNA elongation and reinitiation.

Alternatively, there may be competition between pro- and antiinflammatory transcription factors for limited amounts of cofactors, such as CBP, resulting in a reduction in the expression of inflammatory genes. However, this does not explain the specificity of the cross-talk between the GR and NF-B or between the GR and AP-1. De Bosscher and colleagues have cast further doubt on this hypothesis, showing that overexpression of CBP and other cofactors did not affect the glucocorticoid-concentration response curve (44).

Recent data have suggested a further mechanism for GR action (45). Serine 2 phosphorylation of the C-terminal repeat region of RNA polymerase II, induced by NF-B activity at the IL-8 promoter, is reduced by the GR without affecting the assembly of the preinitiation complex. This again suggests an action for the GR downstream of NF-B DNA binding; however, here it is acting downstream of coactivator function.

Full inflammatory gene expression probably requires that a number of transcription factors act together in a coordinated manner, and repression of a single transcription factor may only partially modify the full response. Glucocorticoids may be able to reduce inflammatory gene expression by repressing a downstream target of transcription factor activation, irrespective of the precise activated transcription factors involved.

The importance of cross-talk in GR actions is indicated by the construction of a GR dimerization-deficient mutant mouse in which the GR is unable to dimerize and therefore bind to DNA, so that the transactivation and transrepression activities of glucocorticoids are separated (46, 47). These animals survive to adulthood, in contrast to GR-knockout animals. In these animals dexamethasone was able to inhibit AP-1–driven and NF-B–driven gene transcription, but the ability to facilitate GRE-mediated effects such as cortisol suppression and T-cell apoptosis was markedly attenuated. This suggests that it will be possible to develop glucocorticoids with a greater therapeutic window.

Effects of Oxidative Stress on GR Function
An important characteristic of the inflammation in COPD is the lack of response to glucocorticoids. Inhaled or oral glucocorticoids have no effect on cell or cytokine profile and do not redress the protease–antiprotease imbalance (48). Alveolar macrophages from patients with COPD also show impairment of the typical ability of dexamethasone to suppress cytokine release, especially release of IL-8 (49).

The lack of response to glucocorticoids has been linked to oxidative stress. Cigarette smoking is the primary cause of COPD, and the smoke contains more than 10¹⁸ oxidant molecules per puff (50). This suggests that oxidative stress may be an important factor in inducing glucocorticoid resistance in COPD. The resistance may be due to cigarette smoking itself, because glucocorticoids are much less effective in reducing inflammatory cells in bronchoalveolar lavage fluid and sputum from smoking patients with asthma compared with nonsmoking patients (51). The resistance to glucocorticoid action is maintained even in subjects who are no longer smoking. This implies that either the oxidant stress is persistent or that the initial chronic insult permanently alters the expression of a component involved in glucocorticoid action. There is good evidence for prolonged persistence of oxidative stress involving long acting lipid peroxidation products, such as 4-hydroxynonenal and isoprostanes, and a reduction in the antioxidant protein gamma-glutamylcysteine synthetase (50).

Okamoto and colleagues (52) have suggested that oxidative stress may influence GR function by inhibiting nuclear translocation of the GR in COS7 and CHO cells. Other potential causes of reduced GR function involve nuclear events. HDAC activity is decreased in bronchoalveolar macrophages and biopsy specimens from smokers and patients with COPD. This decrease in activity correlates with increased inflammatory gene expression and reduced responsiveness to steroids (53). This effect can be mimicked by pretreatment of U937 cells with hydrogen peroxide (100 µM) and parallels the effects seen with the HDAC inhibitor trichostatin A (53). This reduction in HDAC activity by oxidative relates to nitration of tyrosine residues probably within the active site of selective HDACs (54). The results suggest that oxidative stress, by repression of HDAC activity, can modulate GR function in bronchoalveolar macrophages and in U937 cells.

	OTHER APPROACHES TO ANTI-INFLAMMATORY THERAPY

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

 
The elucidation of the molecular mechanisms of glucocorticoids raises the possibility that novel nonsteroidal antiinflammatory treatments might be developed that mimic the actions of glucocorticoids on inflammatory gene regulation. Inhibition of specific HATs activated by NF-B may prove to be useful targets, especially if they also repress the action of other proinflammatory transcription factors (55). Alternatively, activation of HDACs may have therapeutic potential, and theophylline has been shown to have this property, resulting in marked potentiation of the antiinflammatory effects of glucocorticoids both in vitro and in vivo (56). This action of theophylline is not mediated via phosphodiesterase inhibition or adenosine receptor antagonism and, therefore, appears to be a novel action of theophylline (57). It may be possible to discover similar drugs that could form the basis of a new class of antiinflammatory drugs without the side effects that limit the use of theophylline (57).

Many of the antiinflammatory effects of glucocorticoids appear to be mediated via inhibition of the transcriptional effects of NF-B, and small-molecule inhibitors of IKK-2, which activate NF-B, are in development. However, glucocorticoids have additional effects, so it is uncertain whether IKK-2 inhibitors will parallel the clinical effectiveness of glucocorticoids. They may have side effects, such as increased susceptibility to infections; however, as a corollary to this, if glucocorticoids were discovered today, they would be unlikely to be used in humans because of the low therapeutic ratio and their side effect profile. p38 mitogen-activated protein kinase inhibitors also have therapeutic potential as glucocorticoid-sparing agents.

	CONCLUSIONS

TOP ABSTRACT THE MOLECULAR BASIS OF... CHROMATIN REMODELING HISTONE ACETYLATION AND GENE... NF-{kappa}B TEMPORAL ASSOCIATION OF NF... GLUCOCORTICOID-INDUCED GENE... SWITCHING OFF INFLAMMATORY GENES OTHER APPROACHES TO ANTI... CONCLUSIONS REFERENCES

 
Advances in delineating the fundamental mechanisms of gene transcription, especially recruitment of histone-modifying cofactors, have resulted in better understanding of the molecular mechanisms whereby glucocorticoids suppress inflammation. The challenge is to see if these mechanisms hold true in primary cells in vivo. This will undoubtedly lead to the development of drugs that target novel aspects of GR function and potentially restore glucocorticoid sensitivity to diseases that are unresponsive to current therapeutic strategies.

	ACKNOWLEDGMENTS

 
I.M.A. received $500 from GlaxoSmithKline (GSK) in 2002 and 2003 for speaking at conferences organized by GSK, and received $150 from AstraZeneca in 2003 for speaking at a conference organized by them and received research grants from AstraZeneca ($200,000), Boehringer ($300,000), GSK ($300,000), Mitsubishi Pharma ($300,000), and Schering ($30,000); K.I. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript; P.J.B. has previously served as a consultant to GlaxoSmithKline, is a member of scientific advisory boards for GSK, Boehringer Ingelheim, Altana, Pfizer, has received lecture fees from GSK, AstraZeneca, Boehringer Ingelheim and unrestricted grants from GSK, AstraZeneca, Boehringer Ingelheim, Novartis, Millenium, and Scios.

The literature in this area is extensive, and many important studies were omitted because of constraints on space, for which the authors apologize. The authors thank Drs. Borja Cosio and Gaetano Caramori for their helpful discussions.

	FOOTNOTES

 
Work in our laboratories was funded by research grants from AstraZeneca, Boehringer Ingelheim, the British Lung Foundation, the Clinical Research Committee (Royal Brompton Hospital), GlaxoSmithKline, and Mitsubishi Pharma.

(Received in original form February 17, 2004; accepted in final form March 31, 2004)

	REFERENCES

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